2 But why should monads obey these laws?

When we see a program written in a form on the LHS, we expect it to do
the same thing as the corresponding RHS; and vice versa. And in
practice, people do write like the lengthier LHS once in a while.
First example: beginners tend to write

skip_and_get =do{ unused <-getLine
; line <-getLine
; return line
}

and it would really throw off both beginners and veterans if that did
not act like (by law #2)

skip_and_get =do{ unused <-getLine
; getLine}

Second example: Next, you go ahead to use skip_and_get:

main =do{ answer <- skip_and_get
; putStrLn answer
}

The most popular way of comprehending this program is by inlining
(whether the compiler does or not is an orthogonal issue):

Law #3 is amazingly pervasive: you have always assumed it, and you
have never noticed it.

Whether compilers exploit the laws or not, you still want the laws for
your own sake, just so you can avoid pulling your hair for
counter-intuitive program behaviour that brittlely depends on how many
redundant "return"s you insert or how you nest your do-blocks.